go to the admin change form of an instance of a model class with both the "save as" feature enabled, and a TabularInline for relate items.

change the original instance

add a new related instance using the TabularInline

What is the expected output? What do you see instead?

A new instance should be created, a copy of all the related instances should be created and a new instance for any item added in the TabularInline should be created.
Instead the original item is created, all the pre-existing instances are copied but the new ones are not created.

Note: deleting an entry works as expected is just the add that "fails" by creating the new instance without adding the new related items.

Actually, the modification proposed by the OP doesn't seem wrong. Simply removing the BaseInlineFormSet implementation of the total_form_count() method and so leaving to defer to the implementation in BaseFormSet makes the newly filled inline formsets to get validated and (once corrected if they didn't validate) saved associated with the new cloned parent model instance

That if self.save_as_new : condition was added when these methods (total_form_count() and initial_form_count()) were introduced back in r10190 ~two years ago.

Joseph Kocherhans expressed on IRC it is possible this might be an implementation bug.

Changing component because this doesn't seem to be a admin-specific problem.

Not sure why I marked this blocker, regression; on inspection, it doesn't appear to be either. It's been a problem since at least 1.1.X, and it doesn't involve data loss (there's lost data on entry, but nothing destructive happens to existing data)